I need to select the "bundle" entries from the table below but only if a bundle exists other wise select track 1 from each release. Each release has a unique "cat_no" which is why I need to use GROUP BY.

SELECT * FROM cds
WHERE genre='rock'
GROUP BY cat_no
ORDER BY ABS(track) DESC
LIMIT 0,3

This what I have tried but I am sometimes getting track 1 when there is a bundle for that release.